1. Field of the Invention
The present invention relates to a video signal processing method and a video signal processing device each of which is applied to an image-pickup apparatus (e.g., video camera or the like) and performs a predetermined correction process to a luminance signal of a video signal generated by an image pickup operation performed by a solid state image pickup device.
2. Description of Related Art
Conventionally, in a so-called single-plate color video camera using only one sold state image pickup device (charge coupled device: CCD), color filters having different spectral characteristics are arranged on the solid state image pickup device in units of pixels, and signals depending on color components of an object image are obtained from the respective pixels, thereby generating a color video signal. For the video signal generated as described above, color components of the object image are sequentially output according to the arrangement of the color filters, and the color signals are multiplexed. Since the sold state image pickup device can know a specific pixel which a read signal is obtained from, a color signal can be easily separated and demodulated from an output video signal from the solid state image pickup device.
It is an important factor to increase the sensitivity of this single-plate color video camera. Therefore, as color filters employed in the single-plate color video camera, a complementary color filter having transmittance for white light which is higher than that of primary color filters such as R (red), G (green), and B (blue) filters are frequently used.
As the complementary color filter, a filter obtained by arranging Mg (magenta), G (green), Cy (cyan), and Ye (yellow) filters or a filter obtained by arranging W (white), G, Cy, and Ye filters is employed.
In particular, a complementary color filter obtained by arranging W, G, Cy, and Ye filters shown in FIG. 1 is frequently employed in a solid state image pickup device, which is often used in recent years, using a so-called all-pixel reading system in which signal charges of all pixels are independently read for a one-field period and output without mixing them so that high horizontal-and vertical resolutions of an object can be obtained, even if the object is quickly moving.
When the complementary color filter obtained by arranging the W, G, Cy, and Ye shown in FIG. 1 is arranged in a solid state image pickup device which employs the all-pixel reading system as a reading system, a spectral difference may be often generated, as indicated by a hatched portion in FIG. 2, between a luminance signal (Ya=W+G) consisting of W and G and indicated by Ya in FIG. 2 and a luminance signal (Yb=Cy+Ye) consisting of Cy and Ye and indicated by Yb in FIG. 2. It is assumed that the luminance signal Ya is used for a first field and that the luminance signal Yb is used for a second field.
As described above, assume that a spectral difference is generated between the luminance signal Ya consisting of W and G and the luminance signal Yb consisting of Cy and Ye. In this case, thereafter, when the first and second fields are formed by using the luminance signals Ya and Yb later, and an image is displayed on a display device, flickers are generated on the screen of the display device.
Since the spectral difference as described above is caused by a characteristic difference between color filters of a complementary color filter arranged on a solid state image pickup device, it is very difficult (virtually, impossible) to correct the spectral difference by signal processing performed by a circuit arrangement. Therefore, flickers cannot be suppressed.
By the way, the basic processing procedure of a color video camera includes the process of causing a solid state image pickup device to detect an image as charges accumulated in a photoelectric conversion element and the process of causing a low-pass filter to convert a digital signal, obtained by quantizing a quantity of charge, into an analog image signal.
Although a substantial resolution is determined by the number of pixels arranged on a CCD solid state image pickup device, after a sampling process and a restoring process, a false signal caused by an essentially high spatial frequency component is inevitably generated.
FIG. 3 shows an example of a color filter array. Here, a color filter array consisting of W (white), G (green), Cy (cyan), and Ye (yellow) is used, and the four pixels are used as a base unit. In FIG. 3, reference symbol Px denotes a pixel pitch in the horizontal direction, and reference symbol Py denotes a pixel pitch in the vertical direction.
With respect to the horizontal direction, a luminance resolution corresponding a spatial frequency of 1/Px can be expected. However, for example, due to the difference between the intensities of G and Ye, the following false signal is generated when a carrier frequency is represented by fs:(π/2)×(W−G) sin(2π×fs×t)